1//! The chromatic aberration postprocessing effect.
2//!
3//! This makes edges of objects turn into multicolored streaks.
4
5/// See `package::effect_stack::ChromaticAberration` for more
6/// information on these fields.
7struct ChromaticAberrationSettings {
8    intensity: f32,
9    max_samples: u32,
10    unused_a: u32,
11    unused_b: u32,
12}
13
14/// The source framebuffer texture.
15@group(0) @binding(0) var source_texture: texture_2d<f32>;
16/// The sampler used to sample the source framebuffer texture.
17@group(0) @binding(1) var common_sampler: sampler;
18/// The 1D lookup table for chromatic aberration.
19@group(0) @binding(2) var chromatic_aberration_lut_texture: texture_2d<f32>;
20/// The settings supplied by the developer.
21@group(0) @binding(3) var<uniform> chromatic_aberration_settings: ChromaticAberrationSettings;
22
23/// Radial chromatic aberration implemented using the *Inside* technique:
24///
25/// <https://github.com/playdeadgames/publications/blob/master/INSIDE/rendering_inside_gdc2016.pdf>
26fn chromatic_aberration(start_pos: vec2<f32>) -> vec3<f32> {
27    let end_pos = mix(start_pos, vec2(0.5), chromatic_aberration_settings.intensity);
28
29    // Determine the number of samples. We aim for one sample per texel, unless
30    // that's higher than the developer-specified maximum number of samples, in
31    // which case we choose the maximum number of samples.
32    let texel_length = length((end_pos - start_pos) *
33        vec2<f32>(textureDimensions(source_texture)));
34    let sample_count = min(u32(ceil(texel_length)), chromatic_aberration_settings.max_samples);
35
36    var color: vec3<f32>;
37    if (sample_count > 1u) {
38        // The LUT texture is in clamp-to-edge mode, so we start at 0.5 texels
39        // from the sides so that we have a nice gradient over the entire LUT
40        // range.
41        let lut_u_offset = 0.5 / f32(textureDimensions(chromatic_aberration_lut_texture).x);
42
43        var sample_sum = vec3(0.0);
44        var modulate_sum = vec3(0.0);
45
46        // Start accumulating samples.
47        for (var sample_index = 0u; sample_index < sample_count; sample_index += 1u) {
48            let t = (f32(sample_index) + 0.5) / f32(sample_count);
49
50            // Sample the framebuffer.
51            let sample_uv = mix(start_pos, end_pos, t);
52            let sample = textureSampleLevel(
53                source_texture,
54                common_sampler,
55                sample_uv,
56                0.0,
57            ).rgb;
58
59            // Sample the LUT.
60            let lut_u = mix(lut_u_offset, 1.0 - lut_u_offset, t);
61            let modulate = textureSampleLevel(
62                chromatic_aberration_lut_texture,
63                common_sampler,
64                vec2(lut_u, 0.5),
65                0.0,
66            ).rgb;
67
68            // Modulate the sample by the LUT value.
69            sample_sum += sample * modulate;
70            modulate_sum += modulate;
71        }
72
73        color = sample_sum / modulate_sum;
74    } else {
75        // If there's only one sample, don't do anything. If we don't do this,
76        // then this shader will apply whatever tint is in the center of the LUT
77        // texture to such pixels, which is wrong.
78        color = textureSampleLevel(
79            source_texture,
80            common_sampler,
81            start_pos,
82            0.0,
83        ).rgb;
84    }
85
86    return color;
87}